The present invention relates to a wavelength division multiplexing optical fiber amplifier, and more particularly, to a wavelength division multiplexing optical fiber amplifier that amplifies a wavelength division multiplexing light signal using a fiber doped with rare-earth elements.
As a wavelength division multiplexing optical fiber amplifier, EDFA (Erbium Doped Fiber Amplifier) is conventionally known. This EDFA changes output signal optical power according to the number of input signals to keep the input/output level per 1 wavelength constant.
On the other hand, in the field of wavelength division multiplexing optical fiber amplifiers, wavelength division multiplexing optical fiber a amplifiers using a wavelength range of 1580 nm (1570 nm to 1600 nm) instead of the previous 1550 nm range have been developed in recent years. When a high-level (multi-wavelength) signal of approximately xe2x88x925 dBm (32 waves), etc. is input, the efficiency of this wavelength division multiplexing optical fiber amplifier using the 1580-nm range for forward excitation only or for backward excitation only is as low as 10% and achieving output of +20 dBm would require such high excitation power as 1 W. For this reason, improving the efficiency requires bidirectional excitation.
FIG. 4 shows a block diagram of the above-described conventional example of a wavelength division multiplexing optical fiber amplifier using the 1580-nm range. In the figure, signal light to be amplified is introduced from an optical connector 10 through an optical isolator 11a to a wavelength division multiplexing (WDM) coupler 12a. On the other hand, excited light output from an excitation LD (laser diode) light source 13 is bifurcated with a division ratio of 1:1 by a 1xc3x972 division coupler 14 having a division ratio of 1:1.
The signal light is combined with the excited light from the 1xc3x972 division coupler 14 by the WDM coupler 12a, amplified by an erbium-doped fiber (EDF) 15, combined with the excited light by a WDM coupler 12b and output through an optical isolator 11b to an optical connector 16. In this way, this conventional wavelength division multiplexing optical fiber amplifier provides bidirectional excitation by connecting the 1xc3x972 division coupler 14 to the WDM couplers 12a and 12b provided with on the input side and output side, respectively of the EDF 15 and simultaneously executing forward excitation by which the excited light is input from the input side of the EDF 15 in the same direction as that of the signal light and backward excitation by which the excited light is input from the output side of the EDF 15 in the direction opposite to that of the signal light.
However, a number of wavelength sometimes lacks in the wavelength of the conventional wavelength division multiplexing optical fiber amplifier described above, for example, there becomes inputting of approximately xe2x88x9220 dBm (1 wave). In this case, relatively high efficiency is obtained when bidirectional excitation is applied to obtain output of approximately +5 dBm, and therefore output is obtained with a small amount of excitation power. However, since the EDF 15 is a lengthy fiber, the EDF 15 has a poor inverted population due to backward excitation at the incidence end of the signal light, causing possible deterioration of the noise figure.
On the other hand, another conventional optical fiber amplifier is also known (Japanese Patent Application Laid-Open No. HEI 10-209540) that supplies excited laser light to wave combiners connected on both ends of a rare-earth-doped fiber, directly amplifies a light signal input to the one wave combiner and output from the other wave combiner, and includes one excitation laser that outputs excited laser beam and a light division circuit that divides the laser beam input from this excitation laser and supplies the divided laser beams to the wave combiner on the input side and the wave combiner on the output side.
This conventional optical fiber amplifier adjusts, through the light division circuit, the excitation power output ratio of forward excitation to backward excitation to approximately 1:1, but since the EDF is a lengthy fiber as described above, the problem is that making such an adjustment to 1580-nm range signal light will deteriorate the noise figure.
The present invention has been implemented taking into account the points described above and it is an object of the present invention to provide a wavelength division multiplexing optical fiber amplifier capable of providing characteristics such as low noise and high output by basically intensifying forward excitation, and in the case where the output is not obtained by gradually increasing the ratio of backward excitation.
In order to attain the above object, the present invention adopts a configuration comprising:
an optical fiber doped with rare-earth elements that amplifies input signal light;
an excitation LD light source that outputs excited light of a predetermined wavelength;
forward excitation means for supplying the excited light from the signal light input side of the optical fiber;
backward excitation means for supplying the excited light from the signal light output side of the optical fiber;
monitoring means for monitoring power of the output signal light output from the optical fiber; and
controlling means for repeating a first operation that by maximizing the ratio of forward excitation by the forward excitation means relative to the ratio of backward excitation by the backward excitation means, changes the drive current of the excitation LD light source based on a monitor signal from the monitoring means until power of the output signal light reaches a desired value and when power of the output signal light does not reach the desired value, increases the ratio of backward excitation by a predetermined amount relative to the ratio of forward excitation, and a second operation that changes the drive current of the excitation LD light source based on a monitor signal from the monitoring means until power of the output signal light reaches the desired value, until power of the output signal light reaches the desired value.
Since the present invention maximizes the ratio of forward excitation relative to the ratio of backward excitation and changes the drive current of the excitation LD light source in that state, it is possible to provide light amplification focused on forward excitation when input power is low and also provide light amplification focused on forward excitation even when power of the output signal light does not reach a desired value even if forward excitation is intensified, by gradually increasing the ratio of backward excitation.
In order to attain the above object, the present invention is characterized by configuring the above-described controlling means to include:
a directional coupling type optical switch that variably controls the ratio of the excitation LD light source branched to the forward excitation means and the ratio of the excitation LD light source branched to the backward excitation means according to a signal applied to the electrode; and
a control circuit that controls, in an initial state, a signal applied to the electrode of the directional coupling type optical switch so that most of the excited light is supplied to the forward excitation means, compares power of the output signal light detected based on a monitor signal from the monitoring means and the desired value to see whether these two values match or not, changes the drive current of the excitation LD light source until the two values match and adjusts the signal applied to the electrode of the directional coupling type optical switch so that the ratio of the excited light to the backward excitation means increases gradually.
In order to attain the above object, the present invention further provides a wavelength division multiplexing optical fiber amplifier in a two-stage configuration with a first optical fiber amplifier and a second optical fiber amplifier connected via a gain equalizer, the first and second optical fiber amplifiers each comprising:
an optical fiber doped with rare-earth elements that amplifies input signal light;
an excitation LD light source that outputs excited light of a predetermined wavelength;
forward excitation means for supplying the excited light from the signal light input side of the optical fiber;
backward excitation means for supplying the excited light from the signal light output side of the optical fiber;
monitoring means for monitoring power of the output signal light output from the optical fiber; and
controlling means for changing the relative proportion between forward excitation by the forward excitation means and backward excitation by the backward excitation means so that power of the output signal light detected based on a monitor signal from the monitoring means matches a desired value and controlling the drive current of the excitation LD light source,
characterized in that the controlling means of the first optical fiber amplifier controls so that the ratio of the forward excitation becomes relatively greater than the ratio of the backward excitation and the controlling means of the second optical fiber amplifier controls the relative proportion so that power of the output signal light of the second optical fiber amplifier matches a desired value.
The present invention provides an optical fiber amplifier in two-stage configuration, which can, through the first optical fiber amplifier in the first stage that controls the overall noise figure, perform light amplification focused on forward excitation and, through the second optical fiber amplifier in the second stage, perform optimal bidirectional excitation control according to power of input signal light.